The combustion of coal in a boiler, as in a pulverized coal-fired electric power generating plant, produces flyash. The composition of the flyash varies depending, for example, on the composition of the coal and the combustion conditions. Generally, flyash is a fine, solid, noncombustible mineral residue, which is distinct from bottom ash, cinders, or slag. Flyash can have widely varying particle size, density, shape, porosity, internal structure, and surface chemistry. It is typically composed of oxidized silicon, aluminum, calcium, iron, titanium, magnesium, sodium, potassium, sulfur, etc.
The sources of flyash from coal can generally be classified into two categories: mineral inclusions, i.e., extraneous minerals; and, organically associated inorganic elements ("OAI's" or inherent minerals). Inherent minerals are the components of the coal, such as sulfur, sodium, calcium, and potassium, which are not present as mineral inclusions in the coal matrix, but are actually associated with the chemical structure of the complex hydrocarbons which make up the coal's combustible component. The mineral inclusions are the solid, generally crystalline, compounds that are found in salt, rock, clay, and iron pyrites, for example.
The formation of flyash during coal combustion generally depends upon the transformation of minerals during the pyrolytic process of combustion, and the release of inorganic elements on an atomic or near atomic scale from the hydrocarbon matrix that comprises the structure of the coal itself. The inorganic elements released from the organically associated inorganics form a "fume," i.e., a suspension of particles in a gas, with an average particle size of about 1 micron or less. In some instances, the minerals which form the fume will be such as to exist in the vapor state, at least when the fume is the hottest. This may be the case, for example, for sodium and potassium oxides. Typically, the fume is composed of oxides of such elements as sodium, calcium, potassium, and magnesium.
Mineral associated flyash, i.e., ash formed from the mineral inclusions, commonly exists in a fairly wide range of particle sizes. Generally, however, it is most often between about 1 micron and about 100 microns in size (diameter). That is, the particle size distribution of the flyash formed from the mineral inclusions is typically such that the bulk of it, by weight, is of particles about 2 to 70 microns in diameter. In flyash, such materials are often generated from coal as glassy cenospheres.
Disposition of flyash from coal-burning installations such as power generating plants is an increasingly difficult problem. Strict environmental restrictions pertaining to air quality standards and the handling and final placement of flyash have combined to make flyash a source of escalating processing costs and environmental concerns common to nearly all coal-burning plants. To meet the environmental standards, flyash is generally removed from the exiting coal combustion off-gases by such arrangements as scrubbers or baghouses. In a typical example, the gas is fed through a shower of water, such as droplets in a venturi scrubber (or aqueous scrubber). The flyash is collected by the water as the gas passes therethrough. The gas is thereby cleansed and the particulate matter in the water is collected or settled in a pond.
The efficiency of collection of particulate collection equipment is often dependent upon particle size. More specifically, typical particle collection equipment experiences a substantial drop in collection efficiency when the particle size drops below about 1 micron in effective diameter. The drop or "roll off" in efficiency can be substantial. Collection efficiencies exceeding 99% are normal for most devices when collecting particle in a size range from several microns in diameter on up. However, collection efficiencies of about 50% or lower are often experienced for the same equipment when the particle size is smaller than 1 micron. Collection efficiency generally worsens as particle size decreases.
Stack "opacity" is a government regulated flyash emission parameter. It generally concerns definition of the "clarity" of stack emission; i.e., percent transmission through a volume of stack emissions. In general, the greater the opacity, the more contaminated the emissions. (Although smaller particles, i.e., particles of about 1 micron or less in size, may have a disproportionate effect in causing stack opacity.) Extreme stack opacity values can limit the types of coal and/or amount of power that can be produced at a generating unit. That is, certain types of coal cannot be burned without extremely efficient scrubber systems or reduced power output because they generate a large amount of particulate matter, which contributes to opacity. Therefore, some coal-burning facilities are limited in the types of coal that can be burned in order to meet particulate emission standards.
What has been needed is still further systems and methods for reduction in the amount of flyash emissions from combustion processes. In particular, what has been needed is a method to eliminate a substantial portion of ultrafine particulate (about 1 micron or less in diameter) from a combustion gas stream, prior to the gas stream's entrance into a conventional particulate collection device. Thus, the gas stream being emitted to the environment would be largely devoid of both normal fly ash and ultrafine particulate. Such systems and methods would allow a wider range of coals to be burned without penalty, resulting in a more aggressive coal fuel purchasing strategy, and reduced cost of electricity production. The particulate emissions can be reduced; and, the power plant can regain a greater total power output (within opacity limits), if it was "opacity limited." Other advantages may result, such as reduced sulfur, toxic materials and/or flyash output from the plant as a result of the properties of the new coal used.
In addition to flyash emission problems, coal-burning facilities are faced with ash fouling problems. This is because coal-burning facilities have become more efficient by increasing the temperature of the steam produced in the boiler. Boilers, and the tubing (heat exchange surfaces) in the boilers, have also been improved so as not to be the limiting factor in obtaining these high temperatures. However, if the boiler tubes (or heat exchange surfaces) are so hot that they exceed the fluxing temperatures of the flyash which is being transported through the tubes along with the combustion off-gases, the flyash can adhere to the tubes. The flyash deposits can then build up in the tubes and interfere with the movement of off-gases and the rate of steam production. This detrimentally effects the efficiency and capacity of the boiler.
Certain types of coal that produce a relatively low amount of flyash upon combustion can be burned, with concomitant reduction in this ash buildup, i.e., ash fouling, problem. However, this is not always economically efficient. It has also been suggested that vermiculite can be added to the gases and flyash produced during a combustion process. This method, however, does not prevent the formation of flyash deposits. The vermiculite actually combines with the flyash to form ash deposits. Although these deposits are easier to remove than pure ash deposits due to the ability of the vermiculite to expand when exposed to elevated temperatures, they must still be removed by the application of jets of steam or soot blowers. It is, therefore, generally desirable to develop a system and method that reduces the amount of flyash buildup in the boiler system.